Plug an ifaddr leak when changing a route's src

If a route is modified in a way that changes the route's source
address (i.e. the address used to access the gateway), then a
reference on the ifaddr representing the old source address will
be leaked if the address type does not have an ifa_rtrequest
method defined. Plug the leak by releasing the reference in
all cases.

Differential Revision: https://reviews.freebsd.org/D13417
Reviewed by: ae
MFC after: 3 weeks
Sponsored by: Dell